1. Field of the Invention
The present invention relates to a projection exposure apparatus for use in photolithography or the like in an LSI manufacturing process.
2. Description of the Related Art
A conventional projection exposure apparatus is shown in FIG. 10. Light beams emitted from a light source 1, such as a mercury lamp, reach a fly-eye, lens, 3 via a mirror 2 and are divided into a plurality of light beams by the individual lenses 3a which form the fly-eye lens 3. After the light beams have passed through each of the lenses 3a, they each irradiate a reticle via an opening 4a of an aperture member 4, a mirror 5 and a light condensing lens, 6. After the light beams have passed through the reticle 7, each beam reaches a wafer 9 through a projection lens 8. As a result, a pattern formed on the reticle 7 is projected onto the surface of the wafer 9.
When a pattern is exposed using such a apparatus, there are some cases in which the value of the coherency .sigma. of the projection exposure optical system must be made small in order to increase the coherence. Accordingly, in the prior art, as shown in FIG. 11, a plurality of openings, 4a, 4b 4c, having different sizes are formed in the aperture member 4. One opening is selected from among these openings and placed in the optical path, thus adjusting the coherency .sigma.. That is, if the opening 4c having a small opening area is selected, the coherency .sigma. of the optical system becomes smaller, that is, it approaches zero. If the opening 4a having a large opening area is selected, the coherency .sigma. of the optical system becomes larger.
However, if the opening 4c having a small opening area is selected in order to decrease the coherency .sigma., only the illumination light in the peripheral portion of the opening is shielded by the aperture member 4, and thus a problem arises in that the intensity of light at the peripheral portion of the wafer 9 and, thus, the light intensity is not uniform on the wafer.